Antenna radiating elements can be properly matched to the feedline by using a taped loading coil at the feed point of the coaxial feedline. This technique is popular when matching a coaxial feedline to a vertical whip or Marconi type antenna using a ground plane or a vehicle body for the grounded side of the antenna system.
Prior attempts to tune such an antenna to a specific frequency have, insofar as is known, primarily used the following methods, which have been less than entirely satisfactory for various reasons. One prior approach involves connection of a variable capacitor between the antenna radiating element (beyond the loading coil from which the radiating element extends) and ground, commonly a vehicle body. The variable capacitor is then varied by rotating its shaft to tune the antenna. This is a parallel resonance tuned circuit. A disadvantage is that the tuning capacitor requires a weather-proof container to prevent moisture from shorting out its plates and prevent oxidation from degrading the usual rotor connection to ground.
A prior variation has been to couple the ground end of the loading coil through a variable capacitor to the ground plane, or vehicle body. Tuning is then accomplished by rotating the shaft of the variable capacitor. This is a series resonant tuned circuit. This approach also has the disadvantage of requiring a weather-proof container for the variable capacitor to prevent shorting and oxidation as above-mentioned. In addition, the tuning of the series resonant circuit also, and undesirably, affects the impedence match between the coaxial feedline and the antenna.
Prior known attempts to employ capacitive means for antenna tuning, aside from ones of the problems above-outlined, have often also suffered from excessive mechanical complexity and escessive cost, as well as difficulties in adjusting capacitive elements, particularly by hand, when the antenna is radiating power.
Another approach to tuning has been by dematching the antenna and its feedline, such as by moving the feed point on the coil or changing the length of a coil with a shorting device. However, mismatch tuning is an inefficient power radiation technique that often results in radiation of spurious undesirable signals.
A further approach to tuning has been by varying the length of the antenna element above the loading coil. However, the usual means for varying the length of the antenna element grounds out the antenna during tuning and, when the antenna is radiating power, it therefore cannot be tuned. Tuning then becomes a trial-and-error sequence which is both time consuming and potentially inaccurate, involving within each trial-and-error, turning power off and on for adjustment and checking respectively.
Accordingly, the objects and purposes of this invention include provision of:
A tunable loading apparatus particularly for a vertical antenna with an off-center fed base loading coil.
An apparatus, as aforesaid, suitable for Citizens Band and the like communication use and which, for example, permits an antenna to be matched to a 50 ohm coaxial feedline by the position of the feed point on the base loading coil and the antenna to be tuned to the desired specific frequency by manually adjusting a capacitance distributed with respect to the loading coil up and down over the radiating portion of such loading coil.
An apparatus, as aforesaid, permitting adjustment of such distributed capacitance manually and without turning off transmitter power while tuning the antenna, particularly where a relatively low power transmitter, such as a Citizens Band transceiver feeds the antenna, wherein mismatch does not appear as long as tuning is accomplished over a relatively narrow frequency range, wherein weather-proof containers are not required and corrosion does not affect reliable and continued connection or grounding, and further wherein the apparatus is substantially nonaffected by vibration, as in use on automotive vehicles.
An apparatus, as aforesaid, which is simple in construction and to use and can be produced at low cost.
Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.